Superlubricity state through atom-by-atom surface tuning

Friction is everywhere around us, working against motion of cars, airplanes, their engines, wind mills and other devices causing wear and decreasing their energy and overall performance efficiency. However, there exists a certain state, called superlubricity, at which the friction vanishes. This effect can potentially lead to significant improvements in many centuries fight against friction.

It has already been found that the state of the superlubricity can be achieved at the nano-scale between sliding surfaces. Recently, a further step towards understanding the phenomenon was achieved by Alexei Bylinskii and his co-authors at MIT. They showed experimentally that the friction levels can be controlled by tuning the interface of sliding atom-by-atom. In their research, they developed a sub-lattice spatial resolution set up to measure the static friction force and dissipated energy.

The sliding pair consisted of an optical substrate and ion lattice counter object. The optical lattice, was formed by two laser beams forming sinusoidal electric potential. When atoms go through such a field, they take the minimum potential spots and form a periodic grid. To generate the ion-crystal, the ytterbium atoms were ionized using light. The researchers also had to laser cool the ion-crystal to sub-millikelvin temperatures in order to eliminate friction heat effects.

Further, the authors started controlling the atoms spacings by changing the applied laser and light. They further studied parameter q, which was used as an indicator for the degree of matching of the positions of the atoms. When q=1, the atoms spacing completely match, whereas when q=0, there is a complete mismatch. It was then found, that when q = 1, maximum friction occurs. A possible explanation for it is that when the spacing between atoms at the surface of one object is the same or commensurate with the spacing at the surface of the other object, the atoms of one surface become stuck in the minimum potential valleys. For the sliding to start, the applied shear force has to overcome a critical value, so that all the atoms are displaced to the next valley. This also gives a rise to a well-known stick-slip phenomenon. Surfaces match in this case as Lego constructor parts and it gets much more difficult to move them around. On the other hand, when q becomes smaller, the mismatch between the atoms form and fewer atoms fall into minimum potential valleys and friction becomes smaller. That’s why, with the decrease of q, a superlubricity state was observed.

Results of this work may it possible to control friction, at least at nano-scale, by manipulating the structure of the contacting surfaces.